The present invention relates to a tunnel-type reflow furnace, in particular, to a cream solder melting tunnel-type reflow furnace which is furnished with a plurality of heaters on both the top and bottom of the tunnel and which consists of a preheating and a main heating zone. The term "cream solder" as used herein means a creamy substance that is formed by mixing solder in powder form and a liquid flux and which can be deposited in a given amount on selected locations by means of a screen printing apparatus or a dispenser. Such a "cream solder" has been extensively used in soldering electronic components to printed circuit boards.
A reflow furnace as an apparatus for soldering chip-mounting printed circuit boards has been known. Heating means for use in reflow furnaces include a hot gas, infrared radiation (both far and near infrared rays), and even the recently proposed technique of using the latent heat which is liberated when a gas undergoes a phase change to liquid.
In a reflow furnace, selected locations on a printed circuit board where cream solder has been applied are heated to the temperature at which the solder melts, thereby enabling electronic components of interest to be soldered to the circuit board. With modern printed circuit boards on which electronic components are to be mounted, heating of the circuit boards must be held to the necessary minimum level in order to minimize the thermal damage that might be caused to the electronic components. Therefore, an optimum heating pattern or profile does exist depending on the use of a specific printed circuit board.
Conventional reflow furnaces have their own heating profiles and the heating profile of a certain furnace is uniquely determined by its design parameters, namely, it lacks flexibility. The upper and lower limits of heating and cooling temperatures can be adjusted by some degree but time-dependent temperature control has been practically impossible.
With the increasing versatility of electronic components to be mounted on printed circuit boards, it has become desirable to heat circuit boards according to the optimal heating profile of each board. As for the heating of circuit boards themselves, the difference in temperature between the locations where soldering is to be effected and the other areas of the circuit board is preferably as small as possible. However, one is not advised to meet this requirement by simply heating the circuit board over a prolonged period since this will cause thermal damage to the electronic components mounted on the circuit board.
Another tendency in modern electronics industry is to manufacture even lighter and smaller electronic devices and this has increased the need to mount a number of electronic components on a small-area printed circuit board. In soldering printed circuit boards mounting components at high density, the solder applied to very small spaces between individual electronic components must be melted. If heating is performed with a conventional infrared heater, some of the closely packed electronic components will obstruct the travel of infrared radiation which goes on a straight line, and it often fails to travel far enough to reach desired locations where soldering is to be performed. The radiation emitted from a conventional infrared heater has a wavelength of no longer than 3 .mu.m but such infrared rays are not effectively absorbed by a white substance such as solder to realize rapid heating. On the other hand, such infrared rays are rapidly absorbed by dark and sometimes black substances such as electronic components. Therefore, it has often occurred that only the electronic components are heated to high temperatures, with the solder remaining unmelted.